Preheat oven to 375 and spray a 9 x 13 inch pan with non-stick cooking spray. In a medium saucepan, melt margarine, stir in flour, mustard and salt and cook for about 1 minute. Gradually stir in the milk and cook on medium heat, stirring constantly, until mixture thickens slightly (it should coat the spoon). Remove the milk mixture from the heat and stir in 1 1/2 cups cheese and stir until the cheese is melted. Stir in the drained macaroni and then turn into the baking dish. Top with the remaining cheese and bake 20-25 minutes or until bubbly.